Optical or lightguide fibers are inherently versatile as a transmission medium. All forms of information, be it voice, video or data, can be carried on an optical fiber. The most popular medium for lightwave transmission is glass, a solid whose structure is amorphous or random, as opposed to the crystalline structure that normally results when molten materials solidify. Optical fibers for lightwave communications are drawn from a preform which includes an elongated cylinder of glass having an inner core and an outer cladding with the diameters of the core and the cladding being in the same ratio in the fiber as they are in the preform.
In a drawing system, the preform is fed into a heated region where it is necked down to the size of the optical fiber as the fiber is pulled from the heat zone. After the diameter of the optical fiber is measured, one or more protective coatings are applied and the material forming the protective coatings is cured on the drawn fiber.
Typically, the drawn, coated optical fiber is taken up by winding convolutions of the fiber in layers on plastic spools in such a manner that end portions of the fiber on each spool are available for testing. The spools of drawn, tested fiber are used subsequently to supply ribbon and/or cabling processes and apparatus.
The winding during takeup must be carefully controlled, and collection of the fiber at low tension is necessary in order to minimize damage to the fiber or the coating thereon and to reduce the effect of microbending and macrobending losses on the transmission media. Therefore, the winding tension is minimized and the distribution of fiber across a surface of a hub of a spool is controlled to provide a desired profile of the package and to facilitate unwinding in a subsequent operation.
In the prior art, several patterns of winding have been used. First there is a pattern which is referred to as a spiral close wind in which successive convolutions of each layer engage one another. This may result in maximum packing density, but, often times, crossover of one convolution over an adjacent one in the same layer occurs, particularly in view of the very small diameter of the coated fiber, i.e. about 250 microns, and in view of the relatively low tension under which optical fiber is wound during takeup. Such crossovers are referred to as backwinding. Crossovers can lead, undesirably, to loss due to bending.
Steps have been taken to overcome the problem of backwinding. This has been accomplished by spacing apart the convolutions of the wound fiber in each layer. Typically, the spacing has been on the order of the product of about 1.2 to 1.4 and the diameter of the coated optical fiber. However, this remedial approach has led to other problems. It has been found that the resultant package is not as stable as desired and the package deteriorates during the course of normal handling. Also, the density of such a package suffers.
Another problem has been observed in spirally wound packages. Inasmuch as each convolution of the wound fiber is angled to a plane normal to the axis of the spool hub, a mismatch occurs between outer convolutions of each layer and flanges of the spool on which the fiber is wound. As a result, portions of successive layers may drop into gaps between those outer convolutions and the flanges, causing problems during payout.
In order to overcome the problem of mismatch between spool flanges and spirally wound convolutions, the prior art includes a winding pattern which is referred to as an orthocyclic wind. In an orthocyclic close wind, successive convolutions are adjacent to each other but at a predetermined location about the circumference of each of the convolutions, a distributor of the optical fiber is caused to move in a direction generally parallel to the axis of the spool on which the fiber is being wound to cause a jog in each of the convolutions.
Notwithstanding a desire to provide a more stable, low loss package, none of the presently used winding patterns appear to have achieved same. What is sought after and what seemingly has not yet appeared in the prior art is an optical fiber package having excellent stability, high density and low loss. Such a package should be able to be provided on presently available spools with minor modifications of winding apparatus.